_apply_index(): return None when the query string is empty.

I'm unclear whether this is really the right thing, but at least this
prevents crashes when nothing is entered in the search box.

lib/python/Products/ZCTextIndex/ZCTextIndex.py

@@ -118,6 +118,8 @@ class ZCTextIndex(Persistent, Acquisition.Implicit, SimpleItem):
         if record.keys is None:
             return None
         query_str = ' '.join(record.keys)
+        if not query_str:
+            return None
         tree = QueryParser(self.lexicon).parseQuery(query_str)
         results = tree.executeQuery(self.index)
         return  results, (self._fieldname,)